Cidetec surface engineering’s 3R vitrimers: making thermosets circular

Cidetec Surface Engineering has been at the forefront of addressing these issues by developing a new family of advanced thermoset resins called vitrimers, valued for their outstanding versatility.

This innovation is rooted in dynamic covalent chemistry, a key development in the field of thermoset composites. Recognising the need for higher production rates and viable recycling routes in the thermoset sector, Cidetec began studying dynamic covalent systems in 2013. This research led to the development and patenting of a thermo-mechanically reprocessable epoxy resin (EP 3 149 065 B1) that retains the advantages of conventional thermosets while adding 3 new functionalities: reprocessability, reparability and recyclability. The material became known as the 3R Composite.

Since 2015, Cidetec Surface Engineering has collaborated with industrial partners across several key markets, including aerospace, wind energy, automotive and railway. Testing has shown that 3R composites deliver mechanical and thermal performance equivalent to conventional thermosets, making them suitable for existing applications. In addition, all raw materials used are commercially available, which facilitates straightforward industrial adoption.

Dynamic chemistry enabling 3R

Significant efforts have been devoted to developing recyclable, reprocessable and repairable thermosets, epoxies in particular, by introducing reversible bonds into polymer networks. 2 main approaches exist.

• Dissociative CANs: here, bonds break before reforming elsewhere. This temporary loss of crosslink density can cause a sudden drop in viscosity, compromising dimensional stability and solvent resistance. Examples include Diels–Alder systems.
• Associative CANs: bonds exchange without reducing crosslink density. The network remains intact while undergoing topological rearrangements, preserving strength and integrity.

Vitrimers fall into this second category. They are crosslinked networks containing dynamic covalent bonds that exchange without altering crosslink density. At service temperature, they behave like conventional thermosets, with full thermal and mechanical performance. When heated above the so-called topology freezing transition (Tv), bond exchange accelerates, making the material malleable and reprocessable.

Different chemistries can be used to create such dynamic networks, including transesterification, imine exchange or Diels–Alder reactions. Cidetec’s 3R epoxy vitrimers build on this principle using dynamic aromatic disulfide bonds (Ar–S–S–Ar). This associative mechanism provides catalyst-free activation and rapid response under heat, while remaining compatible with standard epoxy–amine formulations. It explains both the high glass transition temperatures achieved and the ability to reprocess, repair and recycle without compromising structural integrity.

These properties define Cidetec ’s 3R technology—Reprocessable, Repairable and Recyclable (Figure 1)

Unlike conventional thermosets, which cannot be reshaped once cured, Cidetec Surface Engineering’s 3R epoxy vitrimers can soften and be reshaped upon heating. This unique property enables manufacturers to repurpose offcuts or adapt existing components without material loss, with an excellent retention of their initial properties.

This was demonstrated through a laboratory-scale validation of the repurposing process of end-of-life (EoL) glass fibre (GF) composite parts, from trimming to manufacturing new parts with different geometries. The study included an accelerated aging stage according to ISO 4892-2, followed by thermoforming-based repurposing of the aged GF/3R epoxy composites. Throughout this process, the 3R epoxy vitrimer matrix remained chemically intact, showing no evidence of degradation or loss of reshapeability, even after aging and multiple processing cycles.

Furthermore, damaged parts can be repaired in place using heat, restoring full mechanical performance and minimising the need to replace entire components due to localised damage.

Selective chemical recycling of 3R vitrimers

3R epoxy vitrimers can be chemically recycled to separate the polymer matrix from the reinforcing fibres. As with conventional epoxy composites, 3R systems show resistance to common chemical media such as THF, toluene, acetone, ethanol, NaOH and HCl. However, the presence of reversible disulfide crosslinks (Ar–S–S–Ar) in the 3R vitrimer network enables selective chemical recycling under appropriate conditions.

The cured resin can be dissolved by solvolysis using a polar aprotic solvent (DMF, NMP or DMSO) in combination with a reducing agent, typically thiols (e.g. acetylcysteine) or disulfides (e.g. diphenyldisulfide). Under moderate conditions (≈60–80 °C, several hours), the matrix degrades while the fibres are released intact. Recovered carbon or glass fibres retain their length and mechanical performance and can therefore be reused in new composite applications.

The patent demonstrates that this approach avoids the fibre surface degradation observed with high-temperature pyrolysis, while requiring much less energy input. The dissolved epoxy matrix can also be recovered and reused. Its composition and properties depend on the recycling conditions, particularly the type and amount of reducing agent. Current research focuses on characterising, optimising and valorising this recycled matrix for second-life applications.

Enduring prepregs

3R epoxy vitrimers can be used to produce enduring prepregs. Traditional prepregs—fibre reinforcements pre-impregnated with resin—typically require cold storage and have a limited shelf life. In contrast, Cidetec Surface Engineering’s enduring prepregs use the patented 3R vitrimer technology to produce semi-cured materials that can be stored at room temperature without loss of properties, including adhesion. This reduces reliance on refrigerated storage and simplifies logistics.

Some formulations have demonstrated a shelf life of more than one year at room temperature, with no detrimental effect on the mechanical performance of the final composite parts. With a glass transition temperature (Tg) between 50 and 70°C, these enduring prepregs soften upon heating, enabling lay-up, draping and consolidation before final curing in an autoclave, press or oven.

They can be processed on standard industrial lines, with only minor adjustments compared to conventional prepreg manufacturing parameters.

Industry-ready compatibility

Beyond their core 3R capabilities, these epoxy vitrimers are designed to integrate seamlessly into existing composite manufacturing processes such as Resin Transfer Moulding (RTM), filament winding and prepreg layup. This drop-in compatibility allows their use without major changes to production infrastructure.

Formulations can be tuned to specific mechanical, thermal and processing requirements as well as lifecycle objectives. As a result, 3R epoxy vitrimers can be applied in sectors including aerospace, railway, wind energy, automotive and hydrogen storage (Table 1).

Table 1: Key characteristics of 3R epoxy vitrimer grades specifically developed for different processes and application sectors.

A track record of R&I leadership

Over the past decade, this 3R epoxy vitrimer technology has progressed through various European and industrial R&I initiatives. One example is the Horizon 2020 Airpoxy project, which demonstrated the scalability of 3R vitrimer formulations for aerospace applications and the ability to reach Tg values of up to 170°C for primary structures. These and other programmes illustrate how this chemistry can be applied across different composite manufacturing routes.

Towards circular composites

Research into vitrimer chemistry has enabled the development of dynamic covalent networks with intrinsic 3R functionalities with high performance epoxy systems. These materials address long-standing limitations of conventional thermosets while maintaining compatibility with industrial-scale manufacturing techniques.

In alignment with the increasing regulatory and market emphasis on circular economy principles, Cidetec Surface Engineering’s 3R epoxy vitrimers provide a technically and economically viable route toward next-generation composite materials with enhanced lifecycle management and reduced environmental footprint.